Chirp controllable Mach-Zehnder optical modulators comprising LiNbO3 (LN-MZ modulators) are in wide use as a component of transmitters for optical communication systems. In recent years, there have been active efforts at development of semiconductor-based Mach-Zehnder optical modulators (semiconductor MZ modulators). A semiconductor MZ modulator can be formed on a semiconductor substrate, such as InP, GaAs, etc., which is used for a semiconductor laser serving as light source. This allows a light source and an optical modulator to be formed on one substrate. The change in refractive index caused by the quantum-confined Stark effect (QCSE) when an electric field is applied to a quantum well structure is larger than the change in refractive index caused by the electrooptical effect of LiNbO3. Thus, the semiconductor MZ modulators are useful for developing small-sized modulators and reducing the driving voltage.
It has been proposed that a Mach-Zehnder optical modulator in which the core layers of the two arms have different thicknesses can operate under either negative or zero chirp conditions. When the absolute value of the bias voltage applied to the arm with the thinner core layer is set to be larger, negative chirp operation can be accomplished. On the contrary, when the absolute value of the bias voltage applied to the arm with the thinner core layer is set to be lower, zero chirp operation can be accomplished.